Hybrid Battery Cycle Life Testing

"Safety was our biggest concern with this flammable system. The remote monitoring and operating capability that this automation system adds to the test eliminated the risk of being around the running system all the time. This also allowed us to place the rugged CompactRIO hardware and the battery system inside a flammable safety cabinet and monitor remotely."

The Challenge:Developing an automation system to evaluate the performance and cycle life of a hybrid lithium-ion/lead-acid battery.

The Solution:Combining CompactRIO FPGAs and processor to create a rugged, reliable automation system that charges/discharges a lithium-ion/lead-acid hybrid battery to evaluate performance and cycle life. The controller monitors voltages, currents, and temperatures of the system and commands the source, load, relay to maintain a continuous charging/discharging cycles in an unsupervised continuous manner. The controller also runs protection algorithm and streams data to the HMI client for logging.

Introduction

A part of the Center for Sustainable Electrical Energy Systems at the University of Wisconsin-Milwaukee, the Power and Electronics and Electric Drives Lab focuses on electrical energy generation and conversion. We integrate multiple energy conversion and storage devices to design systems that provide the most effective, efficient, and reliable means of providing power to loads. These research projects develop technologies for multiple industries and build a talent pipeline for companies in Southeastern Wisconsin.

Battery cycle life testing is time-consuming and the most important procedure in battery qualification test. The battery/system under test is subjected to repeated charge/discharge cycles to determine its cycle life. For our lithium-ion/lead-acid hybrid battery, there are two cycling and capacity check tests. The cycling test includes charging/discharging the combined batteries 1,000 times at a 0.3 C rate. After every 50 cycling tests, we perform a capacity check to measure the capacity of the combined system. The capacity check test includes one cycle of fully charging and discharging the system at 0.1 C. Moving from charging to discharging circuit can take up to one hour and involves disconnecting the source and reconnecting to the load. Additionally, safety hazards arise because we must perform this operation manually and close to the system approximately every three hours.

The goal of the tests is to evaluate the performance and cycle life of the combined batteries to find the feasibility of this combined energy storage for use in multiple utility level applications, including energy harvesting, peak shifting, and frequency regulation. We have conducted several lab scale tests and examined different combined configurations to evaluate the charge and discharge characteristics and cycle performance of the combined batteries.

The result will lead to a formulated estimation about the cycle performance and cycle life of the combined batteries.

System Overview

The combined batteries are seven lithium-ion batteries (LIBs) connected in series and two lead-acid batteries (LABs) connected in series. The seven LIBs are connected in parallel with the two LABs, as Figure 1 shows.

Figure 1. System Overview

Table 1 summarizes the electrical characteristics of each battery cell under test. Figure 2 illustrates the electrical connection of the source and load to the strings along with the voltage, current, and temperature sensors.

Table 1. Electrical Characteristics of the Cells Under the Tests

Figure 2. Electrical Connection of the Source and Load

During the charging cycle, the CompactRIO hardware triggers the relay and the relay energizes the contactor to connect the source to the combined batteries. During the discharging period, CompactRIO discharges the relay, which leads to disconnecting the source from the terminals. Then the digital load is turned on to perform the discharging cycle.

The soft switching path is considered for the first time that the two strings are connected to each other. There is a resistive path along with a circuit breaker to balance the voltage across the two string terminals at the time of connection. Figures 3 through 5 show the system setup.

Figure 3. Automation Controller

Figure 4. System Under Testing With Controller

Figure 5. Hybrid Batter Cycle Life Automation System

Advantages of Using NI Products

Without NI products, we would have to perform many time-consuming and hazardous operations manually. For example, disconnecting the source from the circuit and connecting the load, and vice versa. Programming the source and the load, if needed at each cycle, can be time-consuming. Average testing time for one cycle is seven hours. Handling the circuit exchange manually takes about one hour (two hours per cycle).

We used CompactRIO hardware for system automation. We programmed the FPGA with the NI 9215, NI 9220, and NI 9480 analog input modules for acquiring three different types of signals (voltages, currents, and temperatures). We used LabVIEW software for programming. An NI real-time target runs the control and automation algorithm and communicates with the source and load over serial communication lines using the NI 9870 modules. Data logging runs on the HMI client and communicates with the controller over TCP/IP.

The flexibility to add different modules to CompactRIO, based on the project needs, is what makes it the best candidate at the design stage. LabVIEW block-based programming is easy to learn and does not require extensive programming experience. In addition, we can monitor the whole system remotely, which minimizes the hazard of being around the running system of batteries, which is under intense testing.

Because NI products have been used for years in our lab, NI hardware and programming software became viable platforms in many project setups. We can use FPGA for data acquisition and computational purposes to meet the processing requirements. For example, our system requires data logging every one second. At the same time, data acquisition cannot be interrupted, because CompactRIO is busy with the control algorithm that also includes a hard real-time protection mechanism that requires the system to shut down within milliseconds in case of emergency. CompactRIO was a good choice because the real-time controller receives the data from the FPGA target and runs the algorithm separately, and transmits the collected data to the logging VI running on the computer as a network variable. This makes the system distributed over three different processing units without the interruption.

We programmed our system exclusively with LabVIEW and using the LabVIEW Real-Time and LabVIEW FPGA modules. We programmed the FPGA to acquire continuous data acquisition and faster responses for possible battery faults. Also, because the system runs unsupervised, we programmed it to send any emergency reports through our email server directly from CompactRIO.

Challenges

We encountered some calibration challenges for the NI 9215 and NI 9220 modules, because we are acquiring 9 V from multiple batteries at various levels, two currents, and nine temperatures in addition to the floating nature of our voltage references. We worked closely with NI technical support to overcome this issue by adding grounding points to the system to achieve the highest possible accuracy.

Results

Our system has three major benefits in terms of time, safety, and effort. On average, for a seven lithium-ion and two lead-acid hybrid system configuration, it takes 16 months to run unautomated, assuming that the system has 50 percent probability to finish a cycle outside work hours. Using this automation system, the whole operation takes only nine months running continuously; this saved us seven months of idle system time that can be harmful to the battery system in terms of balancing.

The saved seven months hold approximately three months of continuous labor for replacing the source with the load before the discharging cycle is performed, and the other way around. We saved more effort by using LabVIEW to program the source and the load over RS232 within a second.

Safety was our biggest concern with this flammable system. The remote monitoring and operating capability that this automation system adds to the test eliminated the risk of being around the running system all the time. This also allowed us to place the rugged CompactRIO hardware and the battery system inside a flammable safety cabinet and monitor remotely.

This system can be transformed into multiple configurations to do various testing on any battery system, regardless of its chemistry, and can be beneficial to the battery manufacturing industry.

We believe the system is significant because of the features added to it. For example, programming the source and the load automatically and having the relay to isolate the charging circuit from the discharging circuit. We also added protection with the emergency system shutdown and the automated email reporting immediately when the system had any possible communication or electrical failures.

Using NI products helped to improve and automate the charging test bed design. We fully automated the data acquisition process and data logging. We used 20 data channels to collect data from cells and perform the protection and safety for each cell and also the combined system.

What's Next

After this successful design, we intend to test other combined configurations. We will design a larger test bed to perform several larger combined cells for different applications.

There will be a charging station in our lab to test all different types and configurations of energy storage from cell level all the way to the utility level.

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